Transistor tone control feedback circuit



y 1961 F. D. WALDHAUER 2,994,040

TRANSISTOR TONE CONTROL FEEDBACK CIRCUIT Filed Aug. 6, 1956 2Sheets-Sheet 1 RE'SPO/VJE OVA/346727907763 OF GENERAL Tall ECW40770167/?6 0/ 715 RESPONSE FREGWEA/C INVENTOR.

July 25, 1961 F. D. WALDHAUER 2,994,040

TRANSISTOR TONE CONTROL FEEDBACK CIRCUIT Filed Aug. 6, 1956 2Sheets-Sheet 2 IN VEN TOR.

United States Patent Office 2,994,040 Patented July 25, 1961 2,994,040TRANSISTOR TONE CONTROL FEEDBACK CIRCUIT Frederick D. Waldhauer, Summit,N.J., assignor to Radio Corporation of America, a corporation ofDelaware Filed Aug. 6, 1956, Ser. No. 602,293 1 Claim. (Cl. 330-28) Thisinvention relates to tone control circuits, and more particularly tovariable tone control circuits suitable for use with audio frequencyamplifying systems employing transistors.

Audio frequency amplifying systems often utilize circuits to modify thefrequency characteristics of signals translated therethrough. Thesecircuits may be of a manually variable type, in which separate controlis exercised over the low or bass frequencies and the high or treblefrequencies. A combination of reactive and resistive impedance elements,introduced into the signal path, may be utilized to accomplish thiscontrol. Certain of the resistive impedance elements are made variableunder manual control in order that the desired frequency characteristicmay be selected.

Circuits of this type are generally of the signal divider type, in whichsignal division is a manually controllable function of frequency. Signaldivision may be introduced directly into the signal path, or may beutilized in a feedback network. The former type of circuit is generallytermed a loss type tone control circuit while the latter is termed afeedback type of tone control circuit.

Simple loss type tone control circuits using various combinations ofresistance-capacitance signal attenuating networks generally requiresource and load impedance that are widely different in value to secureproper circuit operation. In the case of electron tube circuits, it iscommon to use a low impedance source of signal voltage, such as afeedback amplifier stage, which provides a source impedance on the orderof the low thousands of ohms and a load impedance, such as the gridcircuit of a succeeding amplifier stage, which provides a load impedanceon the order of a megohm. Thus the tone control is fed from a lowimpedance source and works into a high impedance load, with the ratio ofsource to load impedance being on the order of 1 to 200 or less. Fortransistor tone control circuits the ratio of source of load impedancemay be interchanged, since the electron tube is essentially a voltageoperated device and the transistor a current operated device, so thatthe ratio of the source to load impedance could be of the order of 200to 1. The transistor tone control circuit is then driven from a highimpedance source and operates into a low impedance load.

Ratios on the order of 200 to 1 between the source and load impedance ina normal transistor amplifer may be difficult to obtain, since the inputimpedance of a graunded-emitter transistor amplifier, which may be theload on the tone control circuit, may be on the order of 1,000 ohms,thus requiring an output impedance on the order of 200,000 ohms from theprevious stage. With normal supply voltages, the load resistor of thedriving stage may generally be limited to values of the order of 10,000ohms, so that the source impedance must be lower than this value and isgenerally much lower than the desired value. This is true even with agrounded-base driving amplifier which provides a sufiiciently highsource irm pedance, but which also has a low value of load resistance.

The lowering of the load impedance into which the a tone control circuitmust Work can be utilized to secure the proper ratio of source to loadimpedance. A grounded-base amplifier stage may thus be used followingthe tone control circuit or network, since the grounded base stage has avery low input impedance. Although such a stage may be designed tosecure the proper ratio of load-to-source impedance, the impedance levelof the entire tone control circuit must be correspondingly lowered tothe extent that the components utilized, particularly the capacitiveelements, become large and costly. Other difficulties are encounteredusing a grounded base stage for this purpose, such as, costly biasarrangements, lack of current gain, and noise problems at low signallevels.

Tone control circuits generally provide variable attenuation of thevarious audio frequencies in the form of a boost or cut of the high andlow frequencies (treble and bass response) with respect to the middleaudio frequencies. The tone control may provide attenuatingcharacteristics in which the low and high frequencies at which the boostand cut begins respectively (i.e., the crossover frequencies) aresubstantially constant at all settings of the controls, and the controlsetting determines only the amount of boost or cut. A flat position forthe tone control, that is, one in which no boost or cut is applied tothe audio signal, is, in this case, critically dependent upon componentvalues. This type of control is known as shelving control. Such shelvingtype tone control circuits may require a large number of components, andthe component values to secure proper operation may be critical. It isdesirable from many standpoints to provide a simpler tone controlcircuit and one in which the frequency at which the boost or cut begins,commonly called the turnover frequency, is variable and one that doesnot limit the amount of boost or cut at any given control setting butonly the frequency at which the boost or cut begins.

It is therefore an object of the present invention to provide animproved tone control circuit for transistor audio frequency amplifierswhich effectively obviates many of the difliculties heretoforeexperienced with the use and operation of such circuits.

It is a further object of the present invention to provide an improvedtone control circuit for effectively utilizing the characteristics oftransistors in a single or unitary circuit with relatively few and lowcost components.

It is another object of the present invention to provide an improvedbass and treble tone control circuit for transistor amplifiers having avariable turnover frequency characteristic and simplified control meanstherefor.

It is another object of the present invention to provide an improvedtransistor tone control circuit having an accurate flat frequencyresponse at the midpoint position of the variable control meanstherefor, and providing high values ofbass and treble boost andattenuation while using components having non-critical values and widetolerances.

In accordance with the invention a tone control circuit is connected tothe output circuit of a common-emitter transistor amplifier stage, andarranged effectively in the form of a signal current divider networkhaving an input terminal connected to the collector of a transistoramplifier stage. One output terminal is provided for the tone controloutput signal current, and a second output terminal is connected toprovide a degenerative feedback current to the base electrode of thetransistor. The current divider network is arranged to provide a currentdivision ratio that is dependent upon signal frequency in apredetermined manner to secure a desired frequency response.Continuously variablel tone control is obtained by'variation in thecurrent division characteristics.

The invention may be further understood when the following descriptionis read in connection with the drawings, in which:

FIGURE 1 is a graph showing curves illustrating certain operationalfeatures of tone control circuits in general;

FIGURE 2 is a schematic circuit diagram of a transistor amplifier havinga tone control circuit in accordance with the invention providingvariable turnover characteristics;

FIGURE 3 is a schematic circuit diagram of a transistor amplifier tonecontrol circuit illustrating another embodiment of the presentinvention;

FIGURE 4 is a schematic circuit diagram of a transis-.

tor amplifier tone control circuit in accordance with yet I anotherembodiment of the present invention;

FIGURE 5 is a schematic circuit diagram of a transistor amplifiercircuit having a tone control circuit in accordance with the presentinvention providing variable turnover at bass frequencies and shelving atreble frequencies; and,

FIGURE 6 is a schematic circuit diagram of a transistor amplifier tonecontrol circuit in accordance with the present invention providing ashelving type tone control I circuit.

Referring now to the drawings, and in particular to FIGURE 1, twofamilies of curves 10 and 12 indicate basically the type of frequencyresponse control that may be provided by loss type tone controlcircuits. The curves of FIGURE 1 show only the bass boostcharacteristics usually provided in the tone control circuits, and forillustration purposes represent the type of action for a bass cut,treble boost, and treble cut characteristics that I the same. This isknown as variable turnover, that is,

the frequency at which bass boost begins is variable. However, thefamily of curves 12 of FIGURE 1 all begin to provide a boost in responseat approximately the same frequency and variation in the tone controlsetting provides variation in the amount of ultimate boost only.

- The family of curves 12 indicate what may be called a shelving type ofcontrol.

The particular type of characteristic, variable turnover or shelving,for any particular tone control circuit which a particular designer mayapply is dependent upon the requirements to be met. However, circuitsproviding the variable turnover characteristic are generally of theswitched-capacitor type which does not provide continuously variablecontrol, while circuits of the shelving type are generally variable bythe use of potentiometers. This invention provides a the continuouslyvariable turnover or the continuously 7. variable shelvingcharacteristic, or a combination of both,

without the use of switched capacitors.

Referring now to FIGURE 2, an input signal is applied to a pair of inputterminals of a transistor amplifier circuit, from which the signal isapplied directly between the base electrode 22 and the emitter electrode24 of a first stage transistor 26, here illustrated as a PNP typejunction transistor.

A biasing resistor 28 is connected between the collector electrode 30and the base electrode 22 of the first transistor 26, and the collectorelectrode 30 is connected through a load resistor 32 to a suitablesource of current, here illustrated as a battery 34 having its positiveterminal connected to a point of fixed reference potential or ground forthe amplifier, and having its negative terminal connected to the loadresistor 32.

It will be seen that a signal applied to the input terminals 20 will beamplified and translated through the transistor 26 and appear as anoutput current at the collector electrode 30.

In accordance with the invention, signals appearing at the collectorelectrode are conveyed through a coupling capacitor 36 to an inputterminal 41. Signals applied to the input terminal 41 are appliedthrough a treble control resistor 42 to a current dividing networkcomprising a pair of capacitors 38 and 40. The first of the pair ofcurrent divider capacitors 38 is connected to a permanent tap on thetreble control resistor 42 while the second current divider capacitor 40is connected to a variable tap or slider 44 on the resistor 42. Thesecond capacitor 40 is directly connected to an output terminal 45 whichis connected to the base electrode 46 of a second transistor 48, againillustrated as a PNP junction transistor, while the first capacitor 38is directly connected to a feedback terminal '43 which is connected tothe base electrode 22 of the first transistor 26. Connected in parallelwith the current divider capacitors 38 and 40 are three resistors,namely, a bass control resistor 50 and a pair of limit resistors 52 and54. A control tap or slider 51 on the bass control resistor 50 isconnected to the coupling capacitor through the treble control resistor42.

The emitter electrode 56 of the second transistor 48 is connecteddirectly to ground and the collector electrode 58 is connected to thesource energizing current, the battery 34, through a collector loadresistor 60. Bias is supplied for the base electrode 46 through a biasresistor 62 connected between the collector electrode 58 and the baseelectrode 46. Output signals from the collector electrode are coupledthrough a coupling capacitor 64 to one of a pair of signal outputterminals 68, the other of which is connected directly to ground.

In operation, the current divider capacitors 38 and 40, with the sliders44 and 51 set in the mid-position, provide a current division that isflat with respect to frequency. Attenuation is provided in two ways, byactual division of the output current of the transistor 26 to apply onlya small portion of the output current of the first transistor 26 to thebase electrode 46 of the second transistor 48, and by feedback of theremainder of the output current of the transistor 26 to its own baseelectrode 22 to provide signal degeneration. The current division anddegeneration provide attenuation of all frequencies equally over theentire audio frequency band when the sliders 51 and 44 on the bass andtreble control resistors and 42 are set in their mid-positions.

The feedback over the first transistor 26 also raises 7 the outputimpedance of the transistor 26 to provide a proper source impedance forthe tone control circuit as hereinbefore mentioned. The source impedancemay be shown to be equal to the parallel combination of the collectorjunction resistance of the transistor 26, the reactance of the collectorjunction capacitance of the transistor 26, the bias resistor 28, and thevalue of the load resistor 32 multiplied by the current gain between thebase and collector of the transistor 26 (ea The important feature is thefact that the effect of the load resistor 32 in lowering the outputimpedance is reduced by the factor of the current gain, Dt of thetransistor 26.

The values of the capacitors 38 and 40 are selected so that the desiredamount of attenuation is obtained, considering both the hereinbeforementioned actual division of the output current, and the feedback. Theamount of attenuation required, of course, is dependent upon the amountof boost that is to be made available at the extreme high and lowfrequencies with respect to midband frequencies.

Thus, as an example, it may be desirable to provide approximately 20decibels of attenuation provide a current attenuation of 10 to 1.

in order that approximately 20 decibles of boost may be applied. Theimpedance of the current divider capacitors 38 and 40 should then have aratio such that the effect of the current division and the feedback, incombination, The exact value of the current divider capacitors 38 and 40will depend upon the transistor characteristics in particular thecircuit and the remaining circuit values, but may typically have aratioof 5:1 respectively, the second capacitor 40' having the largerimpedance.

At the high audio or treble frequencies the reactance of the capacitors38 and 40 are low compared to the base control potentiometer 50 so thatat high frequencies the base control path may be treated essentially anopen circuit. To provide treble boost, that is, less attenuation of theextreme high frequencies, the slider 44 is moved from its mid-position,which is at the permanent tap position, toward the upper extremity ofthe resistor 42, as viewed in the drawing. It will now be seen thatthere is a substantial resistance in series with the first currentdivider capacitor 38. As the frequency increases, the impedance of theoutput path of the current divider will become low relative to thefeedback path, so that more high frequency current will flow to the base46 of the second transistor and less will be fed back. This actionproduces a boost at the high frequencies. The turnover frequency, or thefrequency at which an appreciable boost begins, is primarily determinedby the relationship between the impedance of the resistance in serieswith the first capacitor 38 and the reactance of this capacitor, andthus varies with the setting of the slider 44 on the resistor 42.

In a like manner, the treble frequencies may be cut or attenuated agreater extent by moving the slider 44 toward the lower extremity of theresistor 42 thus placing resistance in series with the second currentdivider capacitor 40. Since the impedance of the first current dividercapacitor 38 will decrease as the frequency increases and the resistancewill not decrease, more current is fed back through the first capacitor38 to the base electrode 22 and less current is applied to the baseelectrode 46 of the second transistor 48. This action will provide agreater attenuation or cut in the extreme treble frequencies. Theturnover frequency will depend upon the ratio of the value of theresistance in series with the second capacitor 40 to the impedance ofthe first capacitor 38 and will, of course, vary with the setting of theslider 44.

At base frequencies, the reactance of the current divider capacitors 38and 40 becomes large compared to the resistance of the bass controlresistor 50, and control of the bass frequencies is thus possible bymoving the slider 51 on the bass control resistor 50. To provide a bassboost, that is, less attenuation of the extreme bass frequencies, theslider 51 is moved toward the right extremity of the bass controlresistor 50, as viewed in the drawing. It will now be seen that thesecond current divider capacitor 40 is shunted by a lower resistancethan with the slider 51 in its mid position and more bass current willthus flow to the base electrode 46 of the second transistor and lessbass current will be fed back to the base electrode 22 of the firsttransistor 26. This of course, produces a boost in the bass frequencies,with the turnover frequency, that is the frequency at which asubstantial boost begins, determined by the relationship of the value ofthe resistance in shunt with the second current divider capacitor 40 andthe reactance value of this capacitor. Thus, variation of the positionof the bass control slider 51 to the right of its mid position willprovide a bass boost with a variable turnover fre quency.

Conversely, bass frequency cut will be provided if the bass controlslider 51 is moved from its mid position toward the left extremity ofthe resistor 50. The opposite action will occur to that previouslyexplained with respect to the bass boost circuit, that is, lessresistance will be shunted across the first current divider capacitor38- and more current will be fed back to the base electrode 22 of thefirst transistor 26 and less current will be applied to the baseelectrode 46 of the second transistor 48. This action will produce basscut by further attenuating the extreme bass frequencies. The point atwhich bass cut becomes appreciable, that is, the turnover frequency,

will be determined by the relation between the reactance of the firstvoltage divider capacitor 38 and the resistance in shunt therewith.

The base control potentiometer may have a logarithmic or asemi-logarithmic variation of resistance with shaft rotation, so that atits mid-point of rotation, the resistance shunting each of the currentdivider capacitors 3'8 and 40 will be in approximately the same ratio astheir reactances.

The position of the slider 44 on the treble control resistor 42 willhave no appreciable effect on the bass control characteristics since theresistance value of the treble control resistor 42 is very smallcompared to the resistance of the bass control resistor 50 and thereactance of the current divider capacitors 38 and 40. The

, limit resistors 52 and 54 connected to either extremity of the basscontrol resistor 50 merely serve to limit the maximum turnover frequencyof the bass control for bass cut or bass boost respectively.

The negative feedback connection of the tone control circuit providestwo functions. First, it raises the output impedance of the first stageto provide a high source impedance for the current divider. Second, itaids in pro viding tone control action by making the overalltransmission characteristic of the first stage vary with frequency inthe desired manner. The negative feedback produces this result becauseit will be seen that not all of the current output of the first stage isfed back tothe input electrode of the stage, since the useful outputcurrent which flows to the base of the second stage is not fed back.Suppose, for example, that bass boost is utilized. Then, at mid and highfrequencies, a small portion of the output current of the first stage isfed to the second stage, and the remainder is fed back to the inputelectrode of the first stage. This feedback has the effect of reducingthe mid and high frequency gain of the first stage. At low frequencies,however, most of the output current of the first stage is fed directlyto the second stage, and little is left for feedback. Therefore, thetransmission characteristic of the first stage is increased over itsvalue at mid and frequencies, thereby aiding the tone control action.

A flat position of the tone control providing equal attenuation at allfrequencies is easily achieved, since there is an individual flatposition for each of the bass and treble controls. Positioning thetreble slider 44 at the permanent tap point on the treble controlresistor 42 gives a flat treble position. The flat bass position is notas easily set; however, its flat position is with the slider 51 set onthe low resistance end of the bass control resistor 50 where a largemovement of the slider 51 produces only a small change in resistance.

The transistors 26 and 48 have been here illustrated as junctiontransistors of the PNP type. It should be understood, however, thatother type transistors may be used and that transistors of an oppositeconductivity type, that is a P type conductivity, may be used. If thetransistors 26 and 48 were of P type conductivity, for example, an NPNjunction transistor, the polarity of the supply voltages would obviouslybe reversed.

Referring now to FIGURE 3, a tone control circuit in accordance with theinvention has the same components and connections as the circuit shownin FIGURE 1, and is adapted to be connected to the input, feedback, andoutput terminals 41, 43, and 45 of FIGURE 1. It will be noted that aslightly different connection of the bass control slider 51 is used. Theslider 51 of the bass control resistor 50' is connected to the slider 44on the treble control resistor 42. The operation of the circuit shown inFIGURE 3 is, however, substantially as described in connection in FIGURE2. The current division is accomplished by the current dividercapacitors 38 and 40, as previously explained. At the treblefrequencies, variation of the position of the slider 44 on the treblecontrol resistor will produce a cut or boost in I treble frequencieswith a variable turnover characteristic.

The action of the bass circuit will be the same as explained withreference to FIGURE 2 and boost or cut of the bass frequencies will beaccomplished in the same manner. At bass frequencies the resistance ofthe entire treble control resistor 42 is small compared to theresistance of the bass control resistor 50, and there is no noticeableeffect upon the operation of the circuit.

Referring now to FIGURE 4, a further variation of a tone control circuitin accordance with the invention and adapted to be connected to theinput, feedback and output terminals 41, 43, and 45 of the amplifiercircuit of FIGURE 1, is the same as that described in FIGURE 3, with theexception that an alternate connection is shown for the circuitcomponents. It will be seen the variable and fixed taps on the treblecontrol resistor 42' are reversed from the arrangement shown in FIGURE 3and that the slider 51 on the bass control resistor 50 is connected tothe fixed tap on the treble control resistor 42.

The operation of the circuit is the same as described with reference toFIGURE 2. Current division of the audio frequencies is provided by thecurrent divider capacitors 38 and 40 and bass or treble control with thevariable turnover characteristic is accomplished by positioning of thesliders 44 and 51 on the treble and bass control resistors 42 and 50.Although the tap connections on the treble control resistor 42 arereversed from that of FIGURE 3, the operation of the circuit is the sameas that of FIGURES 2 or 3, since the difference in impedance levels ofthe circuit at bass and treble frequencies, as previously explained,allows the bass control slider 51 to be connected to any point on thetreble control resistor 42 without altering the operation of the circuitappreciably.

Referring now to FIGURE 5, the transistor audio frequency amplifierincludes the pair of transistors 26 and 48 connected in the same manneras the circuit of FIG- URE 2, with the exception that a modification oftone control circuit is provided in accordance with the invention. Itwill also be noted that the transistors 26 and 48 are illustrated as Ptype or NPN transistors. The use of P type transistors necessitates areversal of the connection of the battery 34 as indicated. No change inthe manner of operation of the amplifier or tone control circuits fromthat of FIGURE 2 results from the use of opposite conductivity typetransistors. The bass control circuit is essentially the same as FIGURE2; that is, the bass control potentiometer 50 is connected at oneextremity through the limit resistor 54 to the output terminal 45- andthus to the base electrode 46 of the second transistor 48, and at itsother extremity through the limit resistor 52 to the feedback terminal43 and thus to the base electrode 22 of the first transistor. The treblecontrol, however, includes, in addition to the current dividercapacitors 38 and 40, a treble control resistor 70 connected seriallytherebetween, with a variable slider or tap 72 on the control resistor70 connected directly to the input terminal 41. A pair of currentdivider resistors 74 and 76 are connected between the variable slider ortap 72 and the ends of the treble control resistor 70.

At base frequencies the operation of this tone control circuit isessentially the same as previously described with reference to the basecontrols shown in FIGURES 2, 3, and 4, that is, the current dividercapacitors 38 and 40 provide the basic attenuation which is varied toprovide bass cut or bass boost by positioning the slider 51 along thebass control resistor 50. The current division is essentially capacitiveat the bass frequencies, since the resistance of the treble controlresistor 70 and its resistance may thus be neglected.

However, at the treble frequencies, the reactance of the current dividercapacitors 38 and 40 becomes sufficiently small to be neglected and theresistance of the bass control circuit is very much larger than thereactance of the treble control circuit and its effect may also beneglected. It will be seen that at treble frequencies the currentdivision is essentially resistive. The feedback path is through thefirst current divider resistor 74 and first current divider capacitor38, the reactance of which is very small to the feedback terminal 43.The output circuit path is through the second current divider resistor76 and the small reactance of the second current divider capacitor 40 tothe output terminal 45. Movement of the slider 72 changes the resistancein parallel with the current divider resistors 74 and 76 and thuschanges the amount of current fed back to the base electrode 22 of thefirst transistor 26 and the amount of current applied to the baseelectrode 46 of the second transistor 48. This current dividing actionis unlike that of the circuits of FIGURES 2, 3, or 4, where the currentdivision is accomplished by a capacitive divider. The treble tonecontrol circuit of FIGURE 5 will thus have substantially the sameturnover frequency for the treble control at all settings of the slider72, since the current division is resistive and hence not frequencysensitive. The position of the slider 72 will determine only the amountof boost or cut that will be given.

Other operational features of the circuit are the same as described inconnection with FIGURES 2, 3, and 4; that is, the current through thefirst voltage divider resistor 74 and capacitor 38 is applied to thebase electrode 22 of the first transistor 26, and the feedback increasesthe output impedance of the transistor 26 in the same manner ashereinbefore described.

Referring now to FIGURE 6, a tone control circuit is adapted to beconnected to the input, feedback, and output terminals 41, 43, and 45 ofeither the circuits of FIGURES 2 to 5. The tone control circuit has bothtreble and bass controls and provides a variable shelving type ofcontrol at both bass and treble frequencies.

The treble control resistor 70 is connected between the feedback andoutput terminals 43 and 45 by a pair of capacitors 80 and 82, and theinput current is applied through the input terminal 41 to the slider 72on the treble control resistor 70. The bass control circuit is alsoconnected between the feedback and output terminals 43 and 45 andincludes a first current divider resistor 84, a bass control resistor 86and a second current divider resistor 88. A variable slider 90 on thebass control resistor 86 is connected directly to the input terminal 41.A pair of capacitors 90 and 94 are connected between the slider 90 andeither end of the bass control resistor 86.

It will be seen that the tone control connection of FIGURE 6 willprovide essentially resistive current division to provide the desiredsignal attenuation at both treble and bass frequencies. At treblefrequencies the capacitors 80, 82, 92, and 94 in the tone controlcircuit will exhibit a very low impedance, and the main current divisionwill be provided by the current divider resistors 84 and 88. Thefeedback current will be coupled through the first current dividerresistor 84 to the feedback terminal 43 from where it is applied to thebase electrode 22 of the first transistor 26. The output current will becoupled through the output terminal 45 to the second transistor 48. Ifthe slider 72 on the treble control resistor 70 is moved to the right,less resistance will be shunted across the second current dividerresistor 88 resulting in an increase in output current at the treblefrequencies thus providing treble boost. The converse is true if thetreble control slider 72 is moved to the left, that is, less resistancewill be shunted across the first current divider resistor 84 thusfeeding back more current and providing treble cut.

At bass frequencies, the capacitors 80, 82, 92, and 94 will exhibit avery high impedance, so that the position of the slider 72 of the treblecontrol resistor will have substantially no eiiect. The majority of theinput current from the input terminal 4 1 at bass frequencies will flowto the variable slider 90 on the bass control resistor 86 and throughthe bass control resistor 86 in two paths. The feedback path at bassfrequencies is through the portion of the bass control resistor 86 tothe left of the slider 90, as viewed in the drawing, and through thefirst current divider resistor 84 to the feedback terminal 43, and thetone control output current will flow through the portion of the basscontrol resistor '86 to the right of the slider 90, through the secondcurrent divider resistor 88 to the output terminal 45. It is apparentthat positioning of the slider 90 on the bass control resistor 86 willalter the division of current between the output and feedback paths toprovide control of the bass frequencies. Thus, moving the slider 90tothe right of its midposition will decrease the resistance of theentire right hand or tone control output path providing more current tothe output terminal 45 and resulting in bass boost. Positioning theslider 90 to the left of its mid-position will decrease the entireresistance of the feedback path and feed back more current to thefeedback terminal 43, thus providing bass cut.

A tone control circuit in accordance with the invention suitable for usein transistor amplifiers is characterized by its simplicity and economy.Fewer components may be required than with present day circuits toprovide a tone control having a variable turnover characteristic. Theconventional variable shelving tone control circuit may also be used.Both types of tone control circuits have improved performance resultingfrom the aid to the signal attenuation provided by the feedbackconnection.

What is claimed is:

A combined bass and treble tone control circuit for a transistor audiofrequency amplifier comprising in combination, a signal amplifier stageincluding a transistor having base, collector, and emitter electrodes;at treble frequency control resistor having two end terminals, a 6 fixedtap, and a variable tap thereon; means for connecting one of said endterminals of said treble control resistor to said collector electrode;means for connecting a first current divider capacitor between saidfixed tap and the base electrode of said transistor; means forconnecting a second current divider capacitor between said variable tapand a tone control output terminal; means for connecting a bassfrequency control resistor having two end terminals and a variable tapthereon between said tone control output terminal and the base electrodeof said transistor; and means for connecting the variable tap on saidbass frequency control resistor to said variable tap of said treblecontrol resistor; the relative impedance values of said current dividercapacitor and bass and treble control resistor being so related toprovide a predetermined mid frequency attenuation of the output currentappearing at said collector electrode and to vary the attenuation a thebass and treble frequencies by positioning of the variable taps on saidbass and treble frequency control resistors. 25

References Cited in the file of this patent UNITED STATES PATENTS2,599,271 Michel June 3, 1952 2,695,337 Burwen Nov. 23, 1954 2,745,907Guttwein et al May 15, 1956 OTHER REFERENCES Shea: Principles ofTransistor Circuits, publ. 1953 by John Wiley & Sons, page 296, Fig.14.10.

